Coded electrical control system for motor operated doors



Aug. 21, 1956 E. w. JOHNSON 2,760,134

CODED ELECTRICAL CONTROL SYSTEM FOR MOTOR OPERATED DOORS Filed June 5, 1953 2 Sheets-Sheet 1 IN VEN TOR. EBA/E57 14K JOI/A SOA/ 214M [aw/W Aug. 21, 1956 E, w. JOHNSON CODED ELECTRICAL CONTROL SYSTEM FOR MOTOR OPERATED DOORS 2 Sheets-Sheet 2 Filed June 5, 1953 INVENTOR. [EA/57 W JOHNSON A7702/VEX5 United States Patent CODED ELECTRICAL CONTROL SYSTEM FOR MOTOR OPERATED DOORS Ernest W. Johnson, Everett, Mass., assignor to National Pneumatic Co., Inc, Boston, Mass., a corporation of Delaware Application June 5, 1953, Serial No. 359,774

6 Claims. (Cl. 318-266) The present invention relates to a coded electrical control system particularly adapted for use in controlling the operation of an automatic door operator or the like.

In numerous installations the occurrence of a given event is desired only when a predetermined number of individual stimuli are received Within a given period of time. Mechanical control systems capable of coding received stimuli and actuating an operating device after a predetermined number of stimuli have been received are fairly common. They usually comprise an electro-magnetically actuated step by step ratchet having a cam moved thereby into operative engagement with a switch or the like only after the ratchet has been moved a predetermined number of steps. Mechanisms of this type are fairly expensive and, because of the moving parts, subject to significant maintenance and repair problems. In addition, they are usually not capable of distinguishing between two stimuli delivered in close succession and two stimuli delivered with a large time delay between them. In order to make this distinction it is necessary to provide a timing device and a resetting connection to the ratchet train which greatly complicates the mechanical structure involved, increases the size of the installation, greatly increases its expense, and also greatly increases its susceptibility to break-down.

One particular installation in which the sensitivity to the time Within which a plurality of stimuli are received is very significant is in the control of garage doors or the like. It has been proposed that opening of those doors be automatically controlled by a photoelectric device upon which the headlights of a car may be flashed. Thus when a person approaches his garage, if his headlights are on they will actuate the photoelectric element to cause the garage door to open or, if the headlights are off, they may be flashed on in order to effectuate the same result. However, this particular method of control has not proved entirely satisfactory because cars of guests or deliverymen often enter the driveway at night with their headlights on, but they are not to enter the garage.

Consequently some sort of coded control system is necessary. This usually takes the form of requiring that some predetermined number of independent actuations of the photoelectric element take place before the door will open. For example, the headlights must be blinked on and oif three times, thus producing three separate energizations of the photoelectric element, before the circuit controlling the door operator will be tripped.

However, without some time sensing instrumentality effective to reset the control system after a short period of time has elapsed, even such a coded control system will produce unwanted openings of the garage door. For example, a deliverymans car entering the driveway at night with headlights on may provide one stimulus, a guests car may provide a second stimulus, and when the guest is ready to leave and turns on his headlights, a third stimulus may be produced which will open the garage doors when they obviously should remain closed.

2 ,760,134 Patented Aug. 21, 1956 ice The sequential entry into the driveway of a plurality of delivery trucks whose lights might sweep across the photoelectric element upon entering and leaving might also cause unwanted opening of the garage door. Other variations in these unavoidable situations will suggest themselves.

According to the present invention a coded control system of an all-electric type is produced which will cause a given event to occur only after a predetermined number of stimuli have been received within a predetermined period of time. The only moving parts are relay or switch contacts, and since the currents in the control system are of minimal magnitude the life of the switch contacts is exceedingly great. The circuits are simple, only a limited number of circuit components are employed, and hence the control system can be installed for a very low cost and will function in a highly effective manner.

The system operates by utilizing two capacitors, the capacitance of the second being several times greater than the capacitance of the first. A D. C. source is provided to charge the first capacitor. The second capacitor is utilized to control the output circuit of the system, as by modifying the bias on the grid of an electron tube. Each time that a stimulus is received by the system, the first capacitor is caused to discharge into the second capacitor. In view of the differences between the capacitances of the capacitors, the voltage to which the second capacitor is charged will be less than the voltage to which the first capacitor was charged. However, as the first capacitor is caused to discharge intothe second capacitor a plurality of times through the reception by the system of a plurality of stimuli, the voltage on the second capacitor is built up, and eventually reaches a value sufficient to modify the output of the system to a predetermined extent and initiate the operation of the control device. The second capacitor is provided with a discharge circuit including a time delay element such as a resistor, permitting the charge on the capacitor to leak off slowly. Consequently, the reception by the system of one or more stimuli, but less than the number of stimuli required to appropriately modify the output circuit of the system, will cause the second capacitor to be insufiiciently charged, and that charge will then leak from the second capacitor, thus resetting the system for subsequent actuation according to the predetermined schedule and without regard to the stimuli previously received.

To the accomplishment of the above and to such other objects as may hereinafter appear, the present invention relates to a coded control system and to the use of that system in controlling the operation of a garage door or the like, as defined in the appended claims and as described in this specification, taken together with the accompanying drawings, in which:

Fig. 1 is a schematic simplified view of one embodiment of the present invention;

Fig. 2 is a complete circuit diagram of the system as applied to control the opening and closing of a garage door; and

Fig. 3 is a circuit diagram of an alternative circuit for one part of the system.

Referring first to Fig. 1, an actuated element A, here shown as taking the form of a photocell 2, controls the current in an actuated circuit B which includes relay coil 4. The circuit arrangement is such that normally the current passing through the relay coil 4 is insufiicient to open switch C, that switch normally connecting a first capacitor D to a D. C. source so that the capacitor D is charged. Whenever the element A is actuated the current in the actuated circuit B increases so that the relay coil 4 causes the switch C to open. At the same time switch E is caused to close and connect the first capacitor D to a second capacitor F the capacitance of which is greater than that of the capacitor D. A discharge circuit G is connected to the second capacitor P so as to permit the charge thereon to leak off at a slow rate. The second capacitor F is operatively connected to the output circuit H in such a way as to cause the magnitude of the current to alter in response to the charge on the capacitor F. The output circuit H includes a relay coil 6, that coil acting upon a motor control switch I so as to cause the motor M to operate whenever the coil 6 is sufliciently energized. Because of the difference between the capacitances of the capacitors D and F and because of the nature and design of the output circuit H, the current in the relay coil 6 will have a value such as to cause the switch I to close only after the first capacitor D has discharged into the second capacitor F a predetermined number of times within a predetermined period. This means that, in the application here specifically illustrated, the car 8 must cause the light 10 from its headlights to impinge upon the photo electric cell 2 a predetermined number of times in rapid succession before the motor M will be energized to open the garage door.

In the embodiment specifically illustrated in Fig. l, the actuated circuit B includes a gas discharge tube 12 with A. C. applied across its plate 14 and cathode 16. The tube 12 is grid controlled, the grid 18 being connected in any appropriate circuit including the photoelectric cell 2 in such a way that when light impinges upon the cell 2 the grid 18 is biased more positively so as to cause the tube 12 to fire, thus energizing relay coil 4 and causing the switch C to open and the switch E to close. Because A. C. is applied across the plate 14 and cathode 16, Whenever the bias on the grid 13 falls below a predetermined value the tube 12 will become non-conductive.

A second gas discharge tube 20 is utilized in the output circuit H, D. C. being applied across its plate 22 and cathode 24. Its grid 26 is connected to the positive side of the second capacitor F. Whenever the capacitor F has been charged to a voltage such that the grid 26 will cause the tube 20 to fire, the relay coil 6 will be energized and the switch I will be closed, thus energizing the motbr M which is connected by the switch I to any appropriate source of power. Since D. C. is applied across the plate 22 and cathode 24 of the tube 21 the tube will continue to pass current even after the bias on the grid 26 has dropped, as it will because of the resistor 28 connected across the second capacitor F and defining the time delay discharging circuit G. Consequently a normally closed switch 30 is provided in the plate circuit of the tube 20, opening of the switch 30 breaking the circuit and causing the tube 20 to become and remain nonconductive until the bias on the grid 26 is once again raised to firing value. The switch 30 is momentarily opened when operation of the motor M is to be terminated.

In Fig. 2, which illustrates one complete control circuit, A. C. from a suitable power source, such as the usual 110 volt supply, is applied across terminals 32 and 34, thus energizing primary 36 of transformer 38. One end of the secondary winding 40 is connected by leads 42 and 44 to one side of the photoelectric cell 2, the other side of the photoelectric cell being connected by lead 46 to the grid 18 of tube 12, the bias of the grid 18 being produced by the resistor 48 connected between the lead 46 and the cathode 16. Leads 50 and 52 connect the cathode 16 to the lower end of the transformer secondary 40, the plate 14 of the tube .12 being connected by lead 54, resistor 56, relay coil 4, lead 58, cover switch 69, and leads 62 and 42, to the upper end of the secondary winding 49. A condenser 64 is connected across the relay coil 4 for conventional reasons. The cover switch is normally closed whenever the cover of the housingin which the system is mounted is in proper position, thus performing a conventional safety function.

A second and smaller secondary winding 66 has its upper end connected by lead 68 to contact 70 normally separated from contact 72, the contact 72 being connected by lead 74 to one side of an indicating lamp 76, the other side thereof being connected by leads 44 and 42 to the lower end of the secondary winding 66.

The plate 22 of tube 20 is connected by lead 78 to relay coil 6, and from there, leads tit) and S2 to one end of rectifier 84, the other end of the rectifier being connected by leads S6, 62 and 42 to the upper end of the transformer secondary 4-0. The cathode 24 of the tube 21) is connected by lead 83 to normally closed switch 30, and from there by the leads 9%, 92, 94, 96 and 52 to the lower end of the secondary winding 41). Because the rectifier 34 is therefore in the plate circuit of the tube 20, D. C. voltage is applied across the cathode 24 and plate 22 thereof.

The grid 26 of the tube 20 is connected by resistor 98 to one side of the second capacitor F, a resistor 23 being connected around that capacitor so as to function as the time delay discharging circuit G thereof. The opposite end of the second capacitor F is connected by means of lead 101) to lead 92, which may be considered as at ground potential, the potential of the cathode 24. The first mentioned end of the second capacitor F is connected by lead 1132 to contact 10.4- of the normally open contact pair 104, 106. Contact 106 is connected, by lead 108 and resistor 110, to one side of the first and smaller capacitor D, the other side of that capacitor being connected to ground via lead 112. Contact 106 forms a part of normally closed contact pair 166, 114, contact 114 being connected by lead 116 to lead 82, which is connected to the high potential side of the rectifier 84. A smoothing filter consisting of capacitor 118 and resistor P0 is connected to the output of the rectifier 84.

Relay coil 4 is active on the switches or relays 7072, 104-196 and 1tl6-114, Fig. 2 showing those switches or contact pairs in the normal position which they assume when the relay coil 4 is insufi'iciently energized. When the coil 4 is sufficiently energ zed, however, contact 7G is caused to engage contact 72, thus completing an A. C. circuit through the pilot lamp 76. At the same time contact 166 is separated from contact 114, thus disconnecting the upper end of the first capacitor D from the source of D. C. potential, the contact 106 engaging the contact 104 so as to connect the upper end of the first capacitor D with the lower end of the second capacitor F, thus permitting the former to discharge into the latter.

The relay coil 6 acts on the contact pair 122, 124. those contacts being normally separated as shown, contact 122 being connected by lead 126 to one A. C. terminal 32 and contact 124 being connected by lead 128 to one side of the double pole double throw switch 131), that switch connecting lead 128 to a selected side of the motor M via lead 132 or 134, the other lead 132 or 134 being connected by the switch and the lead 136 to the other A. C. terminal 34. Consequently, when the relay coil 6 is energized sufficiently, contacts 122 and 124 will become engaged and the motor M will be energized and caused to operate so as, for example, to open or close a garage door, the direction of rotation of the motor M being determined by the position of the switch 139. That switch position can be controlled automatically in any one of a number of conventional ways in order to correspond to the position of the door or other operated element, so that when the door is closed the motor will be caused to rotate to open the door, and when the door is open the motor will be caused to rotate to close it.

The magnitudes of the circuit elements involved are conventional except for the relation between the capacitances of the first and second capacitances D and F. The relation between these capacitances will vary in accordance with the bias required to cause the output tube 20 to pass the amount of current necessary to energize the relay coil 6 and close the contacts 122, 124 and will also be determined by the number of individual stimuli selected for coding purposes. In one installation the first capacitor D has a capacitance of .25 mfd. while the second capacitance F has a capacitance of 1 mfd. The tubes 12 and 20 are of the OA46 type. The transformer 38 produces 170 volts across the secondary winding 40 and 6.3 volts across the secondary winding 66 when 115 volts is applied across the primary 36. The photocell is an RCA 922 type. Resistor 48 has a value of 22 megohms. Resistor 56 has value of 2000 ohms. Capacitor 64 has a value of .8 mfd. Resistor 110 has a value of 25,000 ohms. Resistor 98 has a value of .5 megohm while resistor 28 has a value of 40 megohms. Capacitor 118 is of 8 mfd., and resistor 120 has a resistance of 40,000 ohms.

The operation of the system of Fig. 2 is as follows: The bias is such that when only a normal amount of light impinges on the photocell 2 its resistance is sufficiently high so that the current therethrough is insufiicient, when passing through resistor 48, to produce sufficient bias on the grid 18 to cause the tube 12 to fire. Consequently the contacts 70, 72, 104, 106, 114, 122 and 124 assume their positions shown in Fig. 2 and the first capacitor D is charged because it is connected across a source of D. C. potential provided by the rectifier 84, its upper end being at the higher potential.

Upon an increase in the light impinging upon the photocell 2, as when a cars headlights are illuminated and directed thereupon, the resistance of the photocell decreases, the current passing therethrough increases, and the grid 18 of the tube 12 is biased sufficiently to cause that tube to become conductive. The current passing through the tube 12 energizes relay coil 4, and this has three substantially simultaneous results. First, contacts 70 and '72 are closed, thus completing a circuit to the indicating lamp 76 and causing it to become illuminated. Second, contact 106 is separated from contact 114, thus disconnecting the upper end of the first capacitor D from the high potential side of the D. C. source. Third, contacts 106 and are engaged, thus permitting the first capacitor D to discharge into the second capacitor F, raising the potential of the lower end thereof, its upper end being connected to the low potential side of the D. C. source, which may be considered as ground. Because of the differences in the capacitances of the two capacitors D and F, the lower end of the capacitor F will be placed at a considerably lower potential than was the upper end of the first capacitor D. The grid 26 of the tube 20 will be placed at the same potential as the lower end of the second capacitor F, but this will be insufficient to cause the tube 20 to become conductive. When the external stimulus is removed from the photo cell 2, the bias on the grid 18 of the tube 12 will be removed and the circuit will resume the condition shown in Fig. 2, with the exception, however, that the second capacitor F will be somewhat charged. That charge will tend to leak slowly oit therefrom through the resistor 28.

If now the stimulus is again applied to the photocell 2 within a short period of time, lior example, on the order of seconds, the first capacitor D, having been recharged from the D. C. source, will again be emptied into the second capacitor F, thus raising the potential of the lower end thereof and consequently raising the potential of the grid 26.

Let us assume that even this potential is not sufiicient to cause the tube 20 to fire. Upon removal of the second stimulus the circuit will again assume the condition of Fig. 2. A third stimulus applied within a short period of time, before the charge on the second capacitor F has been permitted to leak oif to any appreciable extent, will again increase the charge on that capacitor and will increase the positive bias of the grid 26. With the circuit values given, it has been found that if three stimuli are applied to the system within a period of ten seconds or so, the second capacitor F will be charged thereby to a 6 potential sufiicient to bias the grid 26 so that the tube 20 will become conductive.

When this occurs the relay coil 6 is energized, the contacts 122 and 124 are engaged and the motor is caused to rotate, the direction of rotation being determined by the position of the operated member, such as a door. Because D. C. is applied across the tube 20, it will continue to be conductive even after the bias on the grid 20 has been dissipated by leakage through the resistor 28. Consequently the motor will continue to rotate until it has reached its new position. At that time, through any appropriate mechanical arrangement, such as a cam or the like, the switch 30 is momentarily opened and then permitted to close. The momentary opening of the switch 30 breaks the circuit through the tube 20 and causes it to become non-conductive until a sufficient positive potential is again applied to the grid 26. When the door reaches its final position any appropriate linkage will reverse the position of the switch 130, so that upon its. next cnergization the motor will be caused to rotate in the o osite direction.

In order to provide for manual energization of the motor through the control system in a non-coded manner, as, for example, from the interior of the garage, the normally open and manually closed switch 138 is provided, one end of which is connected to ground and the other end of which is connected via lead 140 to point 142 between resistors 144 and 146, resistor 146 being interposed between point 142 and ground and resistor 144 being interposed between point 142 and the positive side of the rectifier 84. Closing switch 138 produces a short circuit surge or spike which is transmitted via capacitor 148 to the grid 26 of the tube 20, and this surge or spike, because of the presence of resistor 98 which prevents bypassing thereof, biases the grid 26 sufficiently, and for a suflicient period of time, to cause the tube 20 to pass current and energize the relay coil 6.

Resistors 144 and 146 may have resistance values respectively of one-half and one megohm and capacitor 148 may have a resistance of .0005 mfd.

In the system of Fig. 2 gas discharge tubes 12 and 20 are employed because they are especially well adapted to the system in question. They have the characteristic of passing no current or passing a large amount of current, as controlled by small variations in grid bias. In addition, the output tube 20 is desirably, although not neces sarily, of the gas discharge type because, when once rendered conductive, it will continue to be conductive even though the grid bias disappears. However, it will be understood that other circuit arrangements could well be devised to carry out the same novel mode of operation.

Fig. 3 represents one such modification as applied to the actuated circuit B. Here a vacuum tube 12' of the 6BH6 type is employed, that tube having a plate 14', a cathode 16, a control grid 18' and other electrodes. The transformer 38 has a primary winding 36 to which 110 A. C. volts is applied, a secondary winding 40 having no step-up value and a secondary winding 66 which, as in Fig. 2, produces 6.3 volts for the indicator tube 76. A double diode rectifier tube 150 of the 6H6 type is utilized to produce full wave rectification and the positive and negative terminals 152 and 154 thereof are bridged by a pair of 16 mfd. capacitors 156, 157 joined at point 158. A 1 megohm potentiometer 160 is connected across the terminals 152 and 154, the tap 162 thereof being connected by 10 megohm resistor 164 to the cathode end of the photocell 2, the anode end thereof being connected to the high potential point 152 by means of leads 166 and 168. The resistor 164 is also connected to the control grid 18' of the tube 12' via 2 megohm resistor 170, a .005 mfd. capacitor 172 being connected between the resistor 164 and the cathode 16. The relay coil 4 is in the plate circuit of the tube 12. In the circuit of Fig. 3 changes in potential of the grid 18' will be controlled by variations of current passing through the photocell 2,

7 this in turn being dependent upon the light impinging thereon. The bias on the grid '18 will in turn control current through the relay coil 4.

In the circuits as thus far described, the first capacitor D will remain disconnected from the source of D. C. potential and will remain connected to the second capacitor F for as long as the photocell 2 is illuminated. Hence the effect of daylight on the system must be taken into account and compensated for in some manner, or else the system would be inoperative during daytime hours. This follows from the fact that if daylight is allowed to impinge upon the photocell 2, the tube 12 will remain conducting, the relay coil 4 will remain energized, the switch C will remain open and the switch B will remain closed. There will therefore be no opportunity for the first capacitor D to become recharged and discharged into the second capacitor F a plurality of times.

One way of eliminating this difficulty is to so shield or mask the photocell 2 as to prevent difiused daylight from reaching the cell 2 to any appreciable degree, the cell 2 being made sensitive to illumination oriented directly toward itself and coming only from a very limited sector.

Another way to overcome this problem is to modify the grid circuit of the tubes 12 or 12 in the actuated circuit B so that they become responsive only to changes in the current passed by the photocell 2, and not to the absolute value of that current. For example, as indicated in broken lines in Fig. 2, a condenser 49 may be interposed between the photocell 2 and the grid 18, a resistor 51 being connected between the grid 18 and the cathode 16. The capacitor 49 may have a value of l mfd. and the resistor may have a value of 10 megohms. A comparable circuit arrangement could be utilized in the circuit of Fig. 3, as there shown in broken lines. By using capactive coupling between the photocell circuit and the grid, instead of the direct coupling previously described, the advent of daylight will probably cause no efiect whatsoever on the system because the increase in illumination impinging on the photocell 2 will not be rapid enough to cause a significant change in the bias of the grid 18. Even if such a bias change did take place, it would only be temporary and eventually the system would reset itself and be ready for actuation in accordance with the predetermined schedule of applied stimuli.

The systems here disclosed are all-electronic, have no mechanically moving parts except for relay contacts, and yet have the characteristic that they will cause a desired result to be produced only after a predetermined munber of stimuli have been received within a predetermined period of time, and if fewer than the desired number of stimuli are received within that time, the system will automatically reset itself so as to forget those unused stimuli and be ready to receive and count any new stimuli as they are presented to it.

It will be apparent that numerous changes may be made in the specific circuitry disclosed, in the type of stimulus adapted to be sensed, in the number of stimuli received within a given period of time to which the system is made sensitive, and the nature of the device controlled thereby, all within the spirit of the invention as defined in the following claims.

I claim:

1. In a door operator system comprising a motor adapted, when energized, to open and close a door, means for energizing said motor, control means operatively connected to said motor and said energizing means and efiective when actuated to connect said motor and said energizing means, and actuating means for said control means; said actuating means comprising an actuated element, a first capacitor having a given capacity, charging means operatively connected to said first capacitor for charging the same to substantially the same potential as said charging means, a second capacitor having a given capacity, a

discharging circuit for said second capacitor including a time delay element, an output circuit including an electron tube having electrodes, means including said second capacitor to bias said electrodes, the biasing being such that when said second capacitor is charged to a predetermined degree the output of said tube is altered to a predetermined extent, a circuit including a normally open switch between said capacitors whereby, when said switch is closed, said first capacitor discharges into said second capacitor, and an operative connection between said actuated element and said switch effective to close said switch each time said element is actuated, said electron tube being operatively connected to said control means so as to actuate the latter whenever the output of said tube is altered to said predetermined extent.

2. The door operator system of claim 1, in which said actuated element is a photoelectric device and in which said operative connection comprises a circuit in which said device is connected and the current flow through which varies in accordance with the amount of light impinging on said device.

3. In a door operator system comprising a motor adapted, when energized, to open and close a door, means for energizing said motor, control means operatively connected to said motor and said energizing means and effective when actuated to connect said motor and said energizing means, and actuating means for said control means; said actuating means comprising an actuated element, a first capacitor having a given capacity, means for charging said capacitor, a circuit including a normally closed switch between said charging means and said first capacitor whereby, when said switch is closed, said first capacitor is charged, a second capacitor having a capacity at least approximately X times said given capacity, a discharging circuit for said second capacitor including a time delay element, an output circuit including an electron tube having electrodes, means including said second capacitor to bias said electrodes, the biasing being such that when said second capacitor is charged to a degree approximately X times the charge or" said first capacitor the output of said tube is altered to a predetermined extent, a circuit including a normally open switch between said capacitors whereby, when said switch is closed, said first capacitor discharges into said second capacitor, and an operative connection between said actuated element and said switches efiective to open said normally closed switch and close said normally open switch each time said element is actuated, said electron tube being operatively connected to said control means so as to actuate the latter whenever the output of said tube is altered to said predetermined extent.

4. The door operator system of claim 3, in which said actuated element is a photoelectric device and in which said operative connection comprises a circuit in which said device is connected and the current flow through which varies in accordance with the amount of light impinging on said device.

5. In a door operator system comprising a motor adapted, when energized, to open and close a door, means for energizing said motor, control means operatively connected to said motor and said energizing means and eticctive when actuated to connect said motor and said energizing means, and actuating means for said control means; said actuating means comprising a switch member normally in a first condition and movable to a second condition, an actuated circuit including a photoelectric element and a relay coil for said switch member, an increase in the amount of light impinging on said photoelectric element being effective to raise the current in said relay coil sufiiciently to cause said switch member to move to its second condition, first and second capacitors, the latter having a capacity at least X times that of the former, a source of D. C. voltage, a discharge circuit connected to said second capacitor and including a time delay element, said switch member, when in its first condition, connecting said source and said first capacitor and disconnecting said first and second capacitors, and when in its second condition connecting said first and second capacitors and disconnecting said first capacitor from said source, and an output circuit operatively connected to said second capacitor, the status of said output circuit being controlled by the charge on said second capacitor, said output circuit being operatively connected to said control means and comprising a grid controlled gas discharge tube, said grid being connected to and biased by said second capacitor, said tube being normally biased so as not to pass current but being biased to pass current and thus actuate said control means whenever said second capacitor is charged to a degree approximately X times the charge of said first capacitor, a normally closed switch in the plate circuit of said 10 tube, and means responsive to the position of said door for momentarily opening said switch when said door reaches the limit of its movement in a given direction.

6. The system of claim 1, in which second capacitor has a capacity greater than that of said first capacitor.

References Cited in the file of this patent UNITED STATES PATENTS 2,041,079 Lyle May 19, 1936 2,441,145 Hansen May 11, 1948 2,450,021 Schirmer et a1 Sept. 28, 1948 2,564,062 Herrick Aug. 14, 1951 

